This invention relates to an optical glass and, more particularly, to an optical glass having a low glass transition temperature (Tg) and high refractive index and low dispersion characteristics, having excellent internal quality of glass and being suitable for reheat press molding or precision press molding.
One of characteristics required for an optical glass is an excellent internal quality (striae, bubbles and inclusions). The internal quality depends upon a glass composition in some cases and, in other cases, upon conditions of manufacturing the glass. For producing a highly homogeneous optical glass with a good yield in manufacturing an optical glass, a melting apparatus in which a part or whole of a portion which comes into contact with glass melt is made of platinum or platinum alloy is generally used. For example, a crucible, tank, stirring blade and axis made of platinum or platinum alloy are generally used. When melting of glass raw materials is made with such melting apparatus at a high melting temperature or over a long period of melting time, the amount of platinum melting out into the glass increases with the result that platinum or platinum alloy is recrystallized to precipitate in the glass as inclusions (crystals or devitrification) after supersaturation state, or platinum or platinum alloy comes off from the wall of the melting apparatus and becomes inclusions in the glass.
An optical glass which contains a large amount of rare earth oxides such as lanthanum oxide generally has a poor defoaming property and hence there is general tendency toward taking a long period of melting time for defoaming. As described above, inclusions in the glass increase under such melting condition. Accordingly, optical glasses containing a large amount of rare earth oxides such as lanthanum oxide tend to have insufficient internal quality such as bubbles and inclusions.
For overcoming the above described problem in the composition of glass, it is possible to add, as a refining agent, As2O3 which is very effective as a refining agent so as to defoam the glass at a relatively low temperature and thereby reduce the amount of inclusions. Since, however, As2O3 is a material which is detrimental to the environment, As2O3 has not been used recently.
There are spherical lenses and aspherical lenses as lenses used for constituting an optical system. Many spherical lenses are produced by lapping and polishing glass pressings obtained by reheat press molding glass materials. On the other hand, aspherical lenses are mainly produced by precision press molding, i.e., the method according to which lens performs which have been softened by heating are press molded with a mold having a high precision molding surface and the shape of the high precision molding surface of the mold is transferred to the lens performs.
In producing glass pressings by reheat press molding, a very high temperature is required and, therefore, a heat treating furnace is deteriorated in a relatively short period of time with resulting adverse effect to a stable production. Hence, the lower the viscous flow temperature of glass materials, i.e., the lower the glass transition temperature (Tg), the lower the temperature at which reheat press molding can be made with resulting reduction in the load to the heat treating furnace. It is well known in the art that “viscous flow temperature” is about the same as the glass transition temperature.
In obtaining glass moldings such as aspherical lenses by precision press molding, it is necessary to press lens performs which have been softened by heating in a high temperature environment for transferring the shape of the high precision molding surface of the mold to the lens performs and, therefore, the mold used for such precision press molding is subjected to a high temperature and, moreover, a high pressing force is applied to the mold. Hence, in heating and softening the lens performs and press molding the lens performs, the molding surface of the mold tends to be oxidized or eroded, or a release film provided on the molding surface tends to be damaged with the result that the high precision molding surface of the mold cannot be maintained or the mold itself tends to be damaged. In such a case, the mold must be replaced and, as a result, frequency of replacement of the mold increases and production of products at a low cost in a large scale thereby cannot be achieved. Accordingly, glasses used for precision press molding and glasses for glass performs used for precision press molding are desired to have the lowest possible glass transition temperature (Tg) from the standpoint of preventing such damage to the mold, maintaining the high precision molding surface of the mold for a long period of time and enabling precision press molding at a low pressing force.
For the above described reasons, there has been a strong demand in the industry for an optical glass which has, from the standpoint of usefulness in the optical design, high refractive index and low dispersion characteristics, a low glass transition temperature (Tg) and excellent internal quality.
Particularly, there is a strong demand for an optical glass with high refractive index and low dispersion characteristics having a refractive index (nd) and Abbe number (νd) within a range enclosed with a border line consisting of straight lines connecting point A (nd being 1.75 and νd being 50.0), point B (nd being 1.80 and νd being 46.0), point C (nd being 1.80 and νd being 50.0) and point D (nd being 1.75 and νd being 56.0) in the order of the point A, the point B, the point C, the point D and the point A in x-y rectangular coordinates in which x coordinate represents Abbe number (νd) and y coordinate represents refractive index (nd), including refractive index (nd) and Abbe number (νd) on the border line except for the straight line connecting the point A with the point D and the straight line connecting the point B with the point C (hereinafter referred to as “the specific range”).
Since an optical glass having high refractive index and low dispersion characteristics is useful from the standpoint of the optical design, various glasses have been proposed for a long time in the past.
For example, Japanese Patent Application Laid-open Publication No. 2002-128539 discloses a high refractive index, low dispersion B2O3—Ln2O3 optical glass (where Ln is one or more metal selected from the group consisting of Y, La and Gd). Since, however, glasses having optical constants within the above described specific range which are disclosed specifically in this publication do not contain, or contain only a small amount of alkali components, ZnO or fluorine which is effective for making the glass transition temperature low, the glass transition temperature (Tg) of these glasses is high.
Japanese Patent Application Laid-open Publication No. Sho 53-4023 discloses a high refractive index, low dispersion B2O3—La2O3—HfO2 optical glass. Since, however, this glass contains very expensive HfO2 as an essential component, the production cost of the glass is so high that it is not commercially practicable.
Japanese Patent Application Laid-open Publication No. Hei 8-217484 discloses a B2O3—La2O3—Lu2O3 optical glass. Since, however, this optical glass contains very expensive Lu2O3 as an essential component, the production cost of the glass is so high that it is not commercially practicable. Further, since glasses having optical constants within the above described specific range which are disclosed specifically in this publication do not contain, or contain only a small amount of alkali components, ZnO or fluorine which is effective for making the glass transition temperature low, the glass transition temperature (Tg) of these glasses is high.
Japanese Patent Application Laid-open Publication No. Sho 55-3329 discloses a high refractive index, low dispersion SiO2—B2O3—La2O3—Yb2O3—SnO2 optical glass. Since, however, SnO2 which is an essential component becomes metal tin when the melting atmosphere becomes a reduction state and is alloyed with platinum used in the melting apparatus thereby causing erosion which is likely to lead to leaking of glass. Hence, this glass is not commercially practicable. Further, since glasses having optical constants within the above described specific range which are disclosed specifically in this publication do not contain, or contain only a small amount of alkali components, ZnO or fluorine which is effective for making the glass transition temperature low, the glass transition temperature (Tg) of these glasses is high.
Japanese Patent Application Laid-open Publication No. Sho 56-78447 discloses a high refractive index, low dispersion SiO2—B2O3—La2O3—Yb2O3 optical glass. Since, however, this glass requires a high melting temperature of 1350° C. to 1600° C. in refining and homogenizing the glass, inclusions tend to occur in this glass. Further, although there are examples containing alkali oxides which can be melted at a relatively low temperature among specifically disclosed glasses in this publication, these glasses of the specific examples contain As2O3 which is detrimental to the environment. Furthermore, since glasses having optical constants within the above described specific range which are disclosed specifically in this publication do not contain, or contain only a small amount of alkali components, ZnO or fluorine which is effective for making the glass transition temperature low, the glass transition temperature (Tg) of these glasses is high.
Japanese Patent Application Laid-open Publication No. Sho 52-14607 discloses a high refractive index, low dispersion SiO2—B2O3—La2O3—Gd2O3—ZrO2—Ta2O5 optical glass. Since glasses having optical constants within the above described specific range which are disclosed specifically in this publication do not contain, or contain only a small amount of alkali components, ZnO or fluorine which is effective for making the glass transition temperature low, the glass transition temperature (Tg) of these glasses is high.
Japanese Patent Application Laid-open Publication No. Sho 59-169952 discloses a high refractive index, low dispersion SiO2—B2O3—ZnO—ZrO2 optical glass. However, all glasses but one that are specifically disclosed in this publication contain As2O3 which is detrimental to the environment.
Japanese Patent Application Laid-open Publication No. Sho 57-34044 discloses a high refractive index, low dispersion B2O3—La2O3—Yb2O3 optical glass. Since glasses having optical constants within the above described specific range which are disclosed specifically in this publication do not contain, or contain only a small amount of alkali components, ZnO or fluorine which is effective for making the glass transition temperature low, the glass transition temperature (Tg) of these glasses is high.
Japanese Patent Application Laid-open Publication No. Hei 8-259257 discloses a high refractive index, low dispersion SiO2—B2O3—Li2O—ZnO—La2O3 optical glass. Since glasses having optical constants within the above described specific range disclosed specifically in this publication contain As2O3 and, therefore, this glass is not desirable from the standpoint of environmental protection. Moreover, this glass has the disadvantage of producing inclusions.
Japanese Patent Application Laid-open Publication No. Hei 8-26765, No. Hei 6-305769, No. Sho 60-221338 and No. Sho 56-169150 disclose optical glasses which have a low glass transition point (Tg). However, glasses specifically disclosed in these publications do not have optical constants within the above described specific range and, therefore, cannot satisfy requirements for the optical design described above.
As described above, the known technical literature discloses only glasses which have a low glass transition point (Tg) but fail to satisfy the optical constants within the specific range or glasses which satisfy the optical constants within the specific range but have a high glass transition point (Tg) and, therefore, an optical glass which can satisfy an object of the present invention does not exist.
It is, therefore, an object of the present invention to provide an optical glass which has comprehensively overcome the disadvantages of the prior art optical glasses and has optical constants within the specific range, a low glass transition temperature (Tg), excellent internal quality and is suitable for reheat press molding or precision press molding.